Heterodyne receptor system and arrangement for visualizing optical transmission images

ABSTRACT

The present invention provides a receptor system in which laser light from a laser source is directed onto a sample, the transmitted light from which is photomixed by a half mirror with laser light different in frequency from laser light from a local oscillator source, and the photomixed light is received by a receptor element dividing a light propagating zone into a plurality of sub-zones, said receptor element having an exit end, at which a spatial zone, which is defined between different points and in which interference occurs, is limited within a spatially resolvable minimum unit, to form a Fraunhofer diffraction image, whereby the 0 order diffraction image of the Fraunhofer diffraction image is partly or wholly detected by a photodetector, or alternatively a diffraction image at most n times as large as the 0 order spectrum is detected by the photodetector. By extracting a beat component from the photomixed light in this manner, a transmission image can be separated from scattered components for detection. With light including too many scattered components such as that transmitted through the human body or the like, it is possible to obtain information relating to an absorber. This invention is thus applicable to optical computer tomography, etc.

TECHNICAL FIELD

This invention relates to a heterodyne receptor system capable ofdetecting with high resolution information light buried in scatteredlight and an arrangement capable of visualizing optical transmissionimages.

BACKGROUND TECHNIQUE

When light scatterers like biological tissues are illuminated withlight, the resulting, rectilinearly propagating light could be extractedto some extent in a 180° face-to-face system. As yet, however, thespatial resolving power is not good enough.

A difference in the spatial resolving power between light and X-rayscannot be made up for as yet. The use of light, esp. near infrared rays,however, will be able to construct images of tissue's oxygenconcentrations from hemoglobin in the blood. These will provideinformation different from that obtained with other techniques such as

Now let us assume an object 0 shown in FIG. 22 does not contain too muchscatterers and is relatively close to transparency. Then, light having aspecific wavelength component selected through a filter 340 is directedonto the object 0 from a ring type of slit 341 placed at the focalposition of a lens L₁, so that the enlarged image can be focused onto aplane P through an objective L₂ for observation. The use of the ringtype of slit 341 located at the focal point of the lens L₁ is tantamountto irradiating the object 0 with light in every direction, as shown inFIG. 23, so that images I₁, I₂, and so on, of the object 0 in therespective directions can be observed at once.

Given a 3 to 5-cm thick tissue, we can detect light transmitted throughit. This means that "opto-radiography" can be used for diagnosis. Themammas are relatively uniform in tissue and easy to transmit light, andthe transmitted light is easily detectable (at a thickness up to about 3cm) because of their form. Thus, this technique has long been used forthe diagnosis of breast cancer under the name of "Diaphanography" or"Lightscanning". One such conventional diagnostic system will now beexplained with reference to FIG. 24.

The construction of a conventional system for obtaining a lightabsorption distribution image is illustrated in FIG. 24, whereinreference numeral 401 stands for a scan head, 403 the human body, 405 avideo camera, 407 an A/D converter, 409 a near infrared light framememory, 411 a red light frame memory, 413 a processor, 415 a colorconversion processor, 417 an encoder keyboard, 419 a D/A converter, 412a printer, 423 a TV monitor and 425 a video tape recorder. The spot ofthe human body to be inspected, e.g. the mamma is irradiated and scannedalternately with red light (strongly absorbed in hemoglobin in the bloodin particular) and near infrared light (absorbed in the blood, fluids,fat, etc.) by the scan head 401 through a light guide. As shown, thespot is illuminated with light from below. As a result, the mamma glowsbrightly, and the transmission image is picked up by the video camera401. That image is converted through the A/D convertor 407 into digitalsignals, and the near infrared light and red light are fed in the framememories 409 and 411, respectively, through a digital switch. The ratioof intensities of near infrared light and red light is then computed inthe processor 413, and is further converted into analog signals by colorconversion processing. The resulting light absorption distribution imageis finally observed through a printer, a TV monitor or a video tape.

This system's resolving power is not good enough, because the lightleaving the scan head 401, which is not parallel light, diverges throughthe tissue (the mamma), as much it would be illuminated with the lightfrom a flashlight, and is received by such a two-dimensional receptor asa video camera.

An example of conventional illuminator/receptor systems collimated so asto make improvement in this regard will now be explained with referenceto FIG. 25.

FIG. 25 is a diagrammatic sketch illustrating the construction of aconventional unit for obtaining light absorption distribution imagesusing a collimated illuminator/receptor system.

In this example, laser light emanating from a light source is guidedonto an object 435 to be inspected through an optical fiber 433 forillumination, and the transmitted light is picked up by a fibercollimator 437 and fed into a receptor 443 wherein it is converted intoelectrical signals, which are in turn processed in a computer 451through a preprocessing circuit 445, an A/D converter 447 and aninterface 449. In this case, scanning is carried out while the opticalfibers 433 for illumination are synchronized with the fiber collimator437 for detection by a motor 439, thereby obtaining light absorptiondistribution images of the respective spots of the object, which are inturn observed on a monitor 453.

It is noted that the red light source used is a 633-nm He-Ne laser andthe near infrared light source used an 830-nm semiconductor laser. Withthis diagnostic unit, Jobsis and coworkers reported in 1977 that theysucceeded in detecting light transmitted through the heads of cats orhumans illuminated with near infrared light and the amount of thattransmitted light was found to vary depending upon the animals'respiration. With near infrared light having a wavelength of 700 to 1500nm and a tissue specimen, nearly the size of the head of a cat, thetransmitted light could be well detected at a dose of about 5 mill. Thisdose is greatly safe, because it is about 1/50 or less of the presentsafety standards for laser, or about 1/10 of near infrared light towhich we are now being exposed at the seaside.

Incidentally, when living bodies, etc. are irradiated with light, thetransmitted light is absorbed and scattered by the specimens.

FIG. 26 is the Twersky's scattering theory curves that clarifies therelationship between the absorbance of an erythrocite-suspending fluidand the concentration of hematocrit, and shows the intensity, scatteredand absorbance components of the transmitted light obtained byillumination of laser light having a wavelength of 940 nm.

As can be seen from FIG. 26, the transmitted light has the largescattered component superposed on the absorbance component. Thescattered component, because of being directionality-free, has theproperty of coming to contain scattered light from various spots andmaking optical tomograms blurry. For this reason, mere detection of thetransmitted light does not allow the absorbance component, that is therequired information, to be detected with high accuracy.

FIG. 27 is a diagrammatic sketch illustrating the optical properties ofsuch a specimen as a living body.

Referring here back to FIG. 22, the object 0 contains no scatteringcomponent. In other words, what is observed in FIG. 22 is, so to speak,an originally visible object. A specimen 460 of FIG. 27, that is to beactually observed, should be essentially considered to be made up of aRayleigh scatterer 460a smaller than the wavelength of light; a Miescatterer 460b nearly the size of the wavelength of light; a lighttransmission information carrier 460c that is the target to be observedand absorbs light; a diffuser 460d that diffuses light; a diffractiongrating 460e that gives rise to random diffraction; and so on. When sucha specimen is irradiated with coherent plane waves through a laseroptical system 461, light leaving it comes to contain, in addition tothe transmitted light, the Rayleigh scattered, Mie scattered, diffused,randomly diffracted and other forms of light. So far, it has beenimpossible to detect only the light transmitted through the informationcarrier 460c from all such forms of light.

FIG. 28 is a diagrammatic sketch illustrating a Fresnel diffraction waveproduced by a sinusoidal grating having a finite aperture.

As a plane wave is directed onto the finite aperture, side bands 471 and472 occur in addition to transmitted light 470. With a randomdiffraction grating, therefore, difficulty will be involved in detectingand observing the transmitted light with high sensitivity owing to theinfluence of the side bands.

FIGS. 29A and 29B are diagrammatic sketches illustrating a luminancedistribution found on a plane of view located in opposition to a randomscatterer, when it is illuminated with coherent light.

When such a scatterer as a living body is irradiated with such coherentlight as laser light, a random diffraction image appears on the plane ofview, as shown in FIG. 29a. Then, the transmitted light through thescatterer is focused by a lens L onto the plane of view, as shown inFIG. 29b. However, it is impossible to inspect an image of the spot of aliving body or the like to be observed with high resolution, because arandom diffraction image is superposed on that image.

FIGS. 30A and 30B are diagrammatic sketches showing a luminancedistribution of reflected light that depends on what state a diffusereflection plane is in, with FIGS. 30a and 30b taking the form of polarand rectangular coordinates, respectively.

In FIGS. 30A and 30B, J stands for a luminance distribution of lightreflected from a perfect diffusion plane, G denotes a luminancedistribution of light reflected from a glossy plane, and P indicates aluminance distribution of light reflected from a dim plane. On theglossy plane the luminance distribution shows a peak converging sharplyin the predetermined direction, while on the dim plane the luminancedistribution is diverging. Thus, it turns out that the luminancedistribution varies depending upon what state the plane is in and thatthe accuracy of observation making use of reflected light is largelygoverned by what state the plane is in.

As mentioned above, if tomographic images are observed with coherentlight, it is then impossible to view them with high resolution, sincethe required information light is buried in various forms of scatteredlight.

Having been accomplished with a view to providing a solution to theabove-mentioned problems, the present invention has for its object toprovide a heterodyne receptor system capable of detecting the requiredinformation light from many scattered components with the use of a shortreceptor element, even when information light is buried therein, wherebyoptical tomograms of a living body or the like can be imaged, and anarrangement for imaging optical tomograms.

DISCLOSURE OF THE INVENTION

According to one aspect of this invention, there is provided aheterodyne receptor system characterized by including laser transmittedthrough a sample, mixing means for mixing said laser light with anotherlaser light having a frequency different from that of said laser light,a receptor element which the resulting light enters and divides a lightpropagating zone into a plurality of sub-zones and a detector fordetecting a beat component of the mixed light out of light leaving saidreceptor element,

said receptor element having an exit end, at which a spatial zone, whichis defined between different points and in which interference occurs, islimited within a spatially resolvable minimum unit to detect the beatcomponent of the mixed light.

According to another aspect of this invention, there is provided anarrangement for visualizing optical transmission images, characterizedby including a stage for moving a sample, means for directing one of twolaser beams with a given frequency difference therebetween onto saidsample and mixing the transmitted light with the other laser light, areceptor element which the resulting light enters and include aplurality of divided, light propagating zones and a detector fordetecting a beat component of the mixed light out of light leaving saidreceptor element, means for computing the detected signals, and meansfor displaying the result of said computing, whereby a beat component isextracted from light leaving said sample to visualize an opticaltransmission image.

According to this invention, laser light transmitted through a sample ismixed with laser light different in frequency therefrom, and theresulting light is received by a receptor element which divides a lightpropagating zone into a plurality of sub-zones, each of said sub-zonesbeing limited within a spatially resolvable minimum unit in whichinterference occurs between different points, thereby forming aFraunhofer diffraction image to detect the n order diffraction images atmost. This is because it is difficult to extract the 0 order diffractionimage alone when the magnitude of the 0 order Fraunhofer diffractionimage is no more than 1 mm. For this reason, the size of a pinholethrough which diffraction images are extracted is fixed on the mm order,on which an misalignment of an optical system due to vibration orfluctuation is negligible, thereby extracting a spectrum at most n timesthe 0 order light. Then, the beat component is extracted from the mixedlight to separate a transmission image from scattered components fordetection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic sketch illustrating the construction of theheterodyne receptor system according to this invention,

FIG. 2 is a diagrammatic graph illustrating the principle on whichheterodyning according to this invention is based,

FIG. 3 is a table illustrating the principle on which heterodyningaccording to this invention is based,

FIGS. 4, 5A, and 5B each are a diagrammatic sketch illustrating theprinciple of how to form an image,

FIG. 6 is a diagrammatic sketch illustrating how to form an image withcoherent light,

FIG. 7 is a diagrammatic sketch illustrating how to form an image withincoherent light,

FIGS. 8A and 8B are diagrammatic sketches illustrating Fraunhoferdiffraction images formed with plane and spherical waves,

FIG. 9 is a diagrammatic sketch illustrating how to form an imageaccording to this invention,

FIG. 10A and 10B are diagrammatic sketches illustrating how to extractthe 0 order diffraction image from a Fraunhofer diffraction image,

FIG. 11 is a diagrammatic sketch illustrating an optical system fordetecting the 0 order spectrum by two pinholes,

FIG. 12 is a diagrammatic sketch illustrating a high-directionalityoptical system, the inner surface of which is coated with an absorber,

FIG. 13 is a diagrammatic sketch illustrating one embodiment of thisinvention, wherein the 0 order spectrum is detected by a long-focuslens,

FIG. 14 is a diagrammatic sketch illustrating an embodiment of amicroscopic optical system for optical CT,

FIG. 15 is a diagrammatic sketch illustrating an embodiment of anoptical system of high directionality wherein a bundle of a plurality ofoptical units according to this invention is used,

FIG. 16 is a diagrammatic sketch illustrating the heterodyning systemusing a long-focus lens according to this invention,

FIGS. 17 and 18 are diagrammatic sketches illustrating heterodyning witha microscopic optical system for optical CT,

FIG. 19 is a diagrammatic sketch illustrating an embodiment whereinheterodyning is carried out with a bundle of a plurality of long-focuslens receptor systems,

FIG. 20 is a conceptional sketch illustrating how to observe an image byheterodyning,

FIG. 21 is a diagrammatic sketch illustrating an embodiment of thearrangement for imaging optical tomograms using heterodyning accordingto this invention,

FIGS. 22 and 23 each are a diagrammatic sketch illustrating conventionalmethods of observing optical computer tomograms,

FIG. 24 is a diagrammatic sketch illustrating the construction ofconventional equipment for obtaining a light absorption distributionimage,

FIG. 25 is a diagrammatic sketch illustrating the construction ofanother conventional equipment for obtaining a light absorptiondistribution image,

FIG. 26 is a diagrammatic graph showing the Twersky's scattering theorycurves,

FIG. 27 is a diagrammatic illustration of optical properties of a livingbody,

FIG. 28 is a diagrammatic sketch illustrating a diffraction patternproduced through a finite aperture,

FIGS. 29A and 29B are diagrammatic sketches illustrating randomdiffraction patterns produced through a scatterer, and

FIGS. 30A and 30B are diagrammatic sketches illustrating a reflectionpattern on a diffuser plane.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made to the fundamental principle of thisinvention. This invention is based on the so-called van Cittert-Zerniketherorem that, as shown in FIG. 4, the degree (or the complex factor) ofcoherence--which describes the correlation of vibration between a fixedpoint P₂ and a mobile point P₁ on a plane illuminated with light from aquasimonochromatic, primary light source σ of finite dimensions--isequal to the normalized complex amplitude on the corresponding point P₁in a diffraction image around the point P₂, and that this diffractionimage is formed when that light source is replaced by a diffraction holeidentical in size and form with it and its aperture is converged intoP₂, so that the wavefront amplitude is met by a spherical waveproportional to the intensity of the light source. The image formationequation is derived on the basis of this theorem.

Now let us consider an image in a projection optical system which, forsimplicity, may be dealt with two-dimensionally. As shown in FIG. 5a,assume that light from a minuscule light source dx located at a point Xon a plane σ is coherent and passes through a lens Lc and an object 0 toform a spectrum O(s) on a plane L, the center (0 frequency) of which isfound at X. Since σ and L are shown on the same coordinates X on whichthe origin of O(s) is found, the component capable of passing through Lis a portion of that. As illustrated in FIG. 5a, the pupil function f isthen given by

    f(s)=a (s) e.sup.-i (2π/λ)w(s) (|s|≦1) (1)

wherein a(s) and W(s) stand for the absorption and wavefront aberrationsof the lens, respectively. It is assumed, however, that the origin off(s) in Equation (1) is found on the point of intersection of the pupilwith the optical axis. Thus, the spectrum capable of passing throughf(s) is given by

    O(s-X)f(s)

If the intensity of the point X is 1, then the spectrum passing throughthe pupil is subjected to Fourier inverted transformation by the lens L.In other words, the complex amplitude of the image on the imagewiseplane is given by

    o'(u')=∫O(s-x) f (s) e.sup.2πi u's ds              (2)

Thus, the intensity produced by dx on the imagewise plane is given by

    i(u')dX=|∫O(s-X)f(s)e.sup.2πi u's ds|.sup.2 (3)

Equation (3) may also be interpreted as follows. That is, the complexamplitude o'(u') of the image on the imagewise plane is given by##EQU1## It is noted, however, that the variable s in Equation (4) ischanged to s'. It is also understood that although the pupil function isfinite, it is otherwise O so that the lower and upper limits ofintegration are fixed at ±∞.

If s'-X=f' in Equation (4), then ds'=df'. Thus, ##EQU2## Rewriting thevariable as s" and using s"-X=f", we may rewrite (5) as follows:##EQU3## wherein o'*(u') is the complex conjugate for o'(u') or

    i(u')dX=o'(u') o'* (u')dX                                  (7)

Integration of this with all the effective light source σ(X) gives##EQU4##

Now, substituted (5) and (6) for (7) and then for (8) gives ##EQU5##

Here, separation of the integral calculus containing X gives

    ∫σ(X)f (f'+X)f* (f"+X)dX=T(f',f")               (10)

This T is called the cross modulation coefficient. Substituting T for(9) gives the following image formation equation: ##EQU6##

Equation (11) means that if the object spectrum is expressed as O(s),then the image I(u') is found by integrating the total frequency withrespect to the product of the interference fringe formed by a beatbetween the spectra O(f') and O*(f") multiplied by the weight T(f', f").Referring to T(f',f") that is not a function of f'-f" alone, f' and f"vary with position, even when f'-f" vary with position, even when f'-f"is constant. Thus, the image formation equation represents a nonlinearmapping system to which the same T(f', f") cannot be applied, generallymaking image formation analyses difficult, because T(f', f") varies withf' and f", even when the beat frequency f'-f" =f is constant.

For instance, consider transmission of light through a minuscule hole 3in an object plane Σ_(o), as shown in FIG. 6. The light is focusedthrough a lens system 2 onto an image-forming plane Σ_(i) to give alight intensity distribution which spreads around a certain point onthat plane in the form of a ring. This means that light beams from therespective points of the object interfere with one another on theimage-forming plane. Thus, any imagery analyses cannot be made withoutintegrating all the influences of such interference.

Where a solution of this image formation equation is obtained is:

(a) Incoherent System where σ(x) is infinite

T(f', f") is a function of f=f'-f" alone and has a linear form. Thisfunction T(f) is called a response function. In the case of imageformation by incoherent light, the light is directed through a minusculezone 5 in an object plane Σ_(o) and a lens system 2 onto animage-forming plane Σ_(i) to produce an image on a point 4 thereon, asshown in FIG. 7. In this case, the light intensity on the imagery planeis converged into the point 4, giving a sharp peak. Accordingly, therespective points of the object are imaged on the image-forming planeindependently or without interfering with one another.

(b) Coherent System where σ(x) is a point source of light

The image formation equation (11) can be reduced because T(f',f")=const. This function T(f) is called a response function.

(c) Approximately Linear System

This is a partially coherent system in which an object is substantiallytransparent and faint images or minuscule object points are scattered.In that system much of illumination light passes straightforward throughthe object. Thus, only the zero order spectrum is large in magnitudewhile other higher order spectra are very small, so that the componentof the beat f=f'-f" can be disregarded, enabling an image to beprimarily formed by only the beat components of the spectra f"=0 and f'.Then, since f'=f, the system's mapping characteristics can beapproximately described by f alone.

Incidentally, when laser light has been transmitted through an aperture10 as shown in FIG. 8a, it is considered that in the aperture 10 thereare innumerable point sources 11 of light with respect to the scatteredlight. The transmitted light, on the other hand, diverges in the form ofplane diffraction waves that propagate in the same direction as does theincident light. In other words, the radiation pattern of the scatteredlight takes a spherical form and that of the transmitted light,propagating in the form of plane diffraction waves, is of sharpdirectionality. On a sufficiently spaced-away plane P₃, a Fraunhoferdiffraction image is then found. As shown as transmitted light 17, theplane waves show a intensity distribution in which the 0 order spectrumis very large in magnitude but other higher order spectra are small.Scattered light 18 by a gathering of spherical waves, on the other hand,shows a flat form of intensity distribution, as illustrated. When a lens13 has been placed at an intermediate position in the optical system,however, scattered light 19 also shows a diffraction pattern in whichthe 0 order spectrum is relatively large in magnitude. At this positionat which the Fraunhofer diffraction image is obtained, the scatteredlight is so attenuated, as can be seen from FIG. 8a, that the 0 orderspectrum of the plane wave can be sufficiently increased in magnitude.

For observation of the 0 order diffraction image of a Fraunhoferdiffraction image there are two ways, according to one of which it isobserved afar off while an aperture is irradiated with plane waves.According to the other technique, it is viewed on the focal plane of aconvex lens.

The longer the distance of view, the larger the magnitude of the 0 orderdiffraction image (a first dark ring of the Airy's disk) of a Fraunhoferdiffraction image formed through an aperture (a spatially resolvableminimum unit on an object plane) is proportionally. Thus, it may exceedthe aperture in magnitude. Taking the case of using an aperture ofseveral millimeters to a fraction of millimeter in size in combinationwith an incident wavelength of about 500 nm, however, it is possible toobtain the 0 order diffraction image somewhat smaller than the apertureat the shortest distance at which Fraunhofer diffraction can be observedanyhow.

With a convex lens of several centimeters to a few tens centimeters infocal length and several millimeters to a fraction of millimeter, on theother hand, the 0 order diffraction images of Fruaunhofer diffractionimages formed through that lens may be smaller or larger the lensaperture, although this is dependent upon how to combine the focallength with the aperture's size.

Where the magnitude of the Fraunhofer's 0 order diffraction image is nomore than the order of millimeter, some difficulty is involved inextracting the 0 order diffraction light alone due to the oscillation ofa measuring system or other like factors. For that reason, heterodyningis utilized a means for extracting diffraction images at most n timesthe 0 order to make separation between scattered light and transmittedlight.

Where the Fraunhofer's 0 order diffraction image is no less than theorder of a millimeter, on the other hand, heterodyning is again employedas a means for extracting only the 0 order diffraction light wholly orpartly to make separation between scattered light and transmitted light.In this connection, it is noted that the heterodyne system designed toextract a portion of the 0 order diffraction light is inferior in theminimum detection sensitivity to that designed to extract the 0 orderdiffraction light in its entirety.

Now consider the case where the size of the Fraunhofer's 0 orderdiffraction image is no more than the order of a millimeter. In thiscase, if only the 0 order spectrum of the Fraunhofer diffraction imageis observed as the beat component for heterodyning, it is then possibleto glean much information relative to the object of interest--because ofits enhanced light intensity--and to get rid of scattered componentssubstantially. In addition, it is possible to linearize theabove-mentioned response function and thereby simplify image formationanalyses, because the higher order spectra of plane waves are unlikelyto have an influence upon other positions. Referring now to FIG. 9, alight source σ is spaced away by a distance R from a plane P on which aFraunhofer diffraction image is observable. The intensity of light froma minuscule light source S_(ij) on the plane P is detected only withrespect to P_(ij) in the axial direction corresponding to the sourceS_(ij), and not with respect to other positions such as P₁ and P₂.

For instance, one of Fraunhofer diffraction images formed through around aperture is shown in FIGS. 10A and 10B, in which the wave formsshown by solid and broken lines stand for field and light intensities,respectively.

When the round aperture is defined by a pinhole, such a Fraunhoferdiffraction image as shown in FIG. 10a is observed at a positionsufficiently afar off. As shown in FIG. 10b, this is made up of aplurality of dark rings-- called the Airy's disk--and bright zonesbetween the respective dark rings. An inside zone A of the first darkring or, in a better term, the 0 order spectrum zone, is the brightestof all. Thus, if image observation is made with a slit I whose diametercorresponds to a pinhole diameter equal to n times as large as the widthof the 0 order spectrum or which is n times in magnitude as large as apinhole corresponding to the diameter of the first dark ring, only the 0order spectrum is then detected as the beat component for heterodyning,thereby removing higher order spectra. If such detection is carried outwith respect to the respective points, no interference then occurs atdifferent positions; the van Cittert-Zernike theorem does not hold forimage formation. Thus, where scattered light contains minusculeinformation light as with optical computer tomography, it is possible toseparate only the information light from the scattered light fordetection. It is here understood that the van Cittert-Zernike theoremholds within a pinhole. According to this invention, however, the zoneof which this theorem is true is limited to a spatially resolvableminimum unit.

In the case of a plane wave, the condition for forming a Fraunhoferdiffraction image is represented by

    z>>r.sup.2.sub.max /2λ                              (12)

wherein r is the aperture diameter of a light source and z is thepropagation distance. Thus, a Fraunhofer diffraction image may be formedat such a distance as to meet Equation (12) for heterodyning through apinhole n times as large as the 0 order spectrum thereof.

A Fraunhofer diffraction image formed through a pinhole defined by around aperture is given by ##EQU7## where Dr is the radius of thepinhole, J₁ the Bessel function, λ the wavelength and z the optical axislength. The radius of the first dark ring of the Airy's disk is given asfollows:

    Δρ=0.61×z/Dr

Contained in the first dark ring is 84% of the total quantity of light.Thus, if this is admitted in the first dark ring defined by the pinhole,detection can then be made at a 16% loss of the plane wave. Since aspherical wave is attenuated in inverse proportion to the square ofdistance, on the other hand, it is possible to make observation of animage of high resolution by extracting only the 0 order spectrum of theFraunhofer diffraction image.

Incidentally, achieving extraction of only the 0 order spectrum out ofthe Fraunhofer diffraction image with a pipe comparable in diameter tothe pinhole requires an extremely fine, elongated pipe.

Since the larger the pipe diameter, the smaller the Δρ, the use of anormal lens system results in Δρ being extremely reduced in magnitude.Thus, it is difficult to extract only the 0 order spectrum through thepinhole. For this reason, the 0 order spectrum alone is extracted byheterodying with the use of a pinhole larger in magnitude than the 0order diffraction light. That is to say, the present invention isdesigned such that a Fraunhofer diffraction image of light obtained byphotomixing of laser light transmitted through a sample with light froma local oscillator is observed for the beat component to detect only the0 order spectrum of the Fraunhofer diffraction image with the use of arelatively short receptor system having a relatively large pinholediameter.

FIG. 1 is a diagrammatic sketch illustrating the construction of thisinvention. In this figure, reference numeral 01 stands for a laser lightsource, S a specimen, 02 a local oscillator source of light, 03 a halfmirror, 04 an optical system of high directionality, 05 a photodetectorand 06 a filter.

Referring to FIG. 1, the laser light source 01 and the local oscillatorsource 02 differ in wavelength. Light from the light source 01, whichhas passed through the sample S, is photomixed with light from the localoscillator source 02, and the resulting light is received by the opticalsystem of high directionality to be described later. The optical system04, for instance, includes pinholes P₁ and P₂. The photodetector 05 isused to detect a Fraunhofer diffraction image, and the beat component oflight from both the light source 01 and the local oscillator source 02is detected by the filter 6.

As mentioned below, the amplitude of the signal light of the Fraunhoferdiffraction image is determined by whether the aperture is in round,rectangular or annular forms.

Round aperture: 2J₁ (X)/X

Rectangular aperture: sin X/X

Annular aperture: J_(o) (X)

where J₁ or J₀ is the Bessel function and X is the value determined bywhat optical system is used.

Assuming that the Fraunhofer diffraction images formed on thelight-receiving plane by the light transmitted through the sample andthe light from the local oscillator source are expressed in terms of A₁and A₂ in FIG. 2, then the beat component detected through the filter 06is defined by a hatched region therein. Since the beat component isdetected as the product of the Fraunhofer images A₁ and A₂, it iscorresponding to the area of the overlapping regions of the 0 orderspectra. This area reaches a maximum when A₁ and A₂ are in agreementwith each other, and decreases as the amount of overlapping decreases.Thus, even when the diameter of the pinhole is sufficiently large toextract a diffraction image at most n times in magnitude as large as the0 order spectrum, any higher order component cannot be detected becausethe signals detected are made up of the beat component. The signalintensity of this beat component, which varies depending upon acombination of the aperture geometries for the received light and thelight from the local oscillator, is determined in the form of theproduct of both the amplitudes, as shown in FIG. 3, and reaches amaximum when the aperture geometry for the received light coincides withthat for the light from the local oscillator. For heterodyning, it isthus preferred that both the apertures coincide with each other ingeometery. However, this is a matter of choice that is determined by thepurpose for which measurement is made.

FIGS. 11 to 15 illustrate an optical system of high directionality,which serves as a receptor element for selecting a diffraction image ntimes in magnitude as large as the 0 order spectrum of a Fraunhoferdiffraction image at the exit end thereof.

FIG. 11 illustrates one embodiment of the high-directionality opticalsystem having an aperture according to this invention, which serves as areceptor element for detecting a diffraction image.

Laser light is directed from a light source 20 onto a sample 21, thetransmitted light from which is passed through a slit P₁ and thencethrough a slit P₂ spaced away from it by such a distance I as to satisfyEquation (12), to detect the 0 order light by a detector 23.

Now let us assume that the pinhole diameter D of the slit P₂ can berepresented as the following relation:

    D=2Δρ=1.22×λ1/Dr                    (13)

wherein Dr is the pinhole diameter of the slit P₁, λ is the wavelengthof the laser light and Δρ is the radius of a first dark ring. If λ=500nm, 1=6 m and Dr=1 mm, then D=7.32 mm. With the heterodyning receptorsystem of this invention wherein any higher order component cannot bedetected even when a diffraction image at most n times as large as the 0order spectrum is being extracted, however, the pinhole diameter D maybe several times as large as that value.

FIG. 12 is a diagrammatic sketch illustrating another embodiment of thehigh-directionality optical system, wherein reference numeral 30 standsfor an optical element of high directionality, 33 a light absorber, 35 acore and 37 a cladding.

Referring to this figure, the optical element 30, for instance, may beformed of a linear, elongated, hollow glass fiber, the inner wall ofwhich is coated with such a light absorber as carbon.

Upon incidence of light from an entrance plane 35, a light beam parallelwith the optical axis of the optical element 30 propagates rectilinearlyand leaves an exit plane 37. However, a light beam at an angle with theoptical axis is unlikely to leave the plane 37, because it has impingedupon the wall face and been absorbed therein. Now assuming that D is theaperture diameter of the optical element 30, 1 is the length of theoptical element 30 and λ is the wavelength of incident light. Then therelation 1αDr² /λ holds for the length 1, as detected, of a Fraunhoferdiffraction image formed on the plane 37 with perfect plane waves, sincethe components at angles with the optical axis have been absorbed. Thislength enables the Fraunhofer diffraction image to be viewed.

Now consider incident light having a wavelength of 6328 angstroms by wayof example. If Dr=10 mm, then 1=600 m; if Dr=0.01 mm, then 1=0.6 m; ifDr=0.1 mm, then 1=6 cm; if Dr=0.01 mm, then 1=0.6 mm; if Dr=1 μm, then1=6 μm; and if Dr=0.5 gm, then 1=1.25 μm.

Thus, if the aperture diameter and length of the high-directionalityoptical element are preset depending upon the object to be measured andthat length is sufficiently long relative to that aperture diameter,then it is possible to extract only plane waves parallel with theoptical axis from light waves entering the optical element, In order toachieve substantial plane wave propagation, however, it is required thatthe diameter of the optical element be larger than the wavelength ofincident light. If the optical element should have a diameter nearlyequal to the wavelength of incident light, so large would the amount ofdiffraction be that the quantity of light extracted is extremelyreduced. In this embodiment, the optical element can also be increasedin diameter by using the heterodyne receptor system.

When the plane waves as signal light are detected in the form of the 0order Fraunhofer diffraction image alone, the degree of separation ofincoherent scattered light from plane waves is given by

(Scattering Intensity)/(Intensity of Transmitted Plane

    Waves)=(λ/Dr).sup.2

wherein Dr is the diameter of the entrance aperture of the highlydirectional optical element and λ is the wavelength.

In other words, the larger is the aperture diameter Dr relative to thewavelength, the more the attenuation of scattered light, so that it canbe separated from the plane waves. Even when a Fraunhofer diffractionimage n times as large as the 0 order spectrum is detected through thepinhole, the separating power is on the same level, because the beatcomponent is of the same zero order as mentioned above.

In a modification to FIG. 12 that is contrary to a conventional opticalfiber, the index of refraction of the core section is made smaller thanthat of the rest. Light beams at angles with the optical axis dissipatewithout being subjected to total reflection. Even through a part of themis reflected, all this comes to disappear from the optical element whilereflection is repeated several times. It is thus possible to detect allplane waves but scattered components.

FIG. 13 is a diagrammatic sketch illustrating a further embodiment ofthis invention, in which a lens is used.

Referring to FIG. 13, a lens 25 is used to form a Fraunhofer diffractionimage through an aperture on the front focal plane and focus it onto theback focal plane, whereby the length of a receptor system can bereduced. In this lens system, the aperture diameter D may again be foundby Equation (13). Now consider incident light having a wavelength of 500nm by way of example. If the focal length f=1 m and Dr=1 mm, then D=1.22mm, or if the focal length f=5 m and Dr=5 mm, then D=1.22 mm. The use ofthis embodiment in combination with heterdyning according to thisinvention enables the length of the receptor system to be furtherreduced.

FIG. 14 is a diagrammatic sketch illustrating an embodiment of themicroscopic optical system for optical computer tomography.

Referring to FIG. 14, laser light is converged through a condenser lensL₁ onto a sample 0. The sample is then located in the vicinity of thefront focal point of an objective L₂ for observation on an enlargedscale. In this case, the Fraunhofer diffraction image of the sample 0 isformed through the objective L₂ on the back focal point F. The size ofthe resulting 0 order diffraction image is then considered tantamount toa Fraunhofer diffraction image formed through the objective L₂ whenplane waves enter an aperture equal in size to the 0 order diffractionimage on the focal plane of the condenser lens L₁ (which is found withinthe sample plane). However, this is true when the size of the 0 orderspectrum of the Fraunhofer diffraction image formed on the focal planeof the objective L₂ upon incidence of plane waves on it satisfies thecondition that it must be larger than that of the 0 order spectrum ofthe Fraunhofer diffraction image formed through the condenser lens L₁.

At this time, the size of the 0 order diffraction image exceeds theorder of millimeter. Thus, the incoming light is brought in wavefrontalignment with light from a local oscillator on the focal plane or aposition before or after it, where heterodyning takes place.Alternatively, a reduced image that is the Fourier transformation imageof the 0 order diffraction image is formed on the focal plane through aneyepiece L₃, which is extracted through a pinhole P for heterodyning.The size D of the 0 order diffraction image on this pinhole becomes

    D≈λf.sub.2 /D.sub.2

wherein f₂ and D₂ are the focal length and aperture of the eyepiece L₃,respectively. Thus, D may be made smaller or larger than the size of the0 order diffraction image on the plane 0 of the sample by selecting theF value of the lens L₃ -f₂ /D₂. In order to view a general image of thesample with this embodiment, the sample's plane may be scanned withlaser light. In FIG. 14, broken lines represent optical paths forscattered light, which diffuses in the form of spherical waves and isattenuated.

FIG. 15 is a diagrammatic sketch illustrating an embodiment of thehigh-resolution optical system comprising a bundle of a plurality of theoptical systems of high directionality according to this invention, withwhich a general image of an sample can be viewed at once.

An optical unit 60, for instance, is built of a plurality of suchoptical elements 61 as described in connection with FIGS. 11 to 14, andhas such a length 1 as to satisfy Equation (12). The aperture D of thisunit is such that a diffraction image at most n times as large as the 0order spectrum is extractable from Fraunhofer diffraction images. Byusing such an optical unit in combination with heterodyning, it ispossible to view a clear object image because at the exit end of theoptical unit, the optical elements are independent of each other with nointerference in the respective positions.

FIG. 16 is a diagrammatic sketch illustrating an embodiment of thehigh-directionality optical system comprising a lens and pinholes, whichis used in combination with heterodyning.

Referring to FIG. 16, light form a laser light source 71 is divided by ahalf mirror into two portions. One light portion is directed onto asample S, while the other is photomixed with the light transmittedthrough the sample S by way of a mirror 73, a phase shifter 74 and amirror 75. The light passing through the phase shifter 74 has itsfrequency shifted, and photomixed with the rectilinearly propagatinglight, entering a receptor system through an aperture P₁. A long-focuslens 78 has its front focal plane located at the aperture position, anda Fraunhofer diffraction image formed through the aperture is extractedthrough a pinhole P2 located on the back focal plane of the long-focuslens, the beat component of which is in turn detected by a detector 79.Detection of the beat component is synchronized with the operating cycleof a chopper 77, thereby removing such gradual variations as power ortemperature variations. And the use of the long-focus lens enables thelength of the receptor to be reduced.

FIG. 17 is a diagrammatic sketch illustrating an embodiment of themicroscopic optical system for optical computer tomography.

Referring to FIG. 17, laser light is split into two laser beams by ahalf mirror. One laser beam is converged through a condenser lens L₁onto a sample 0 placed in the vicinity of the front focus of anobjective L₂, while the other laser beam is frequency-shifted by a phaseshifter and photomixed on a half mirror 76 with the light from theobjective. Then, the resulting image is then enlarged through aneyepiece L₃ whose front focus is located at the back focus position ofthe objective L₂, with its beat component being detected through apinhole in a plane P. Now let us assume that f₁ and f₂ represent thefocal lengths of the objective and eyepiece, respectively. Then, theFraunhofer diffraction image observed satisfies the relation f₂ >f₁. Inorder to observe a general image of the sample with this embodiment, thesample's plane may be scanned with laser light.

FIG. 18 is a diagrammatic sketch illustrating another embodiment of themicroscopic optical system for optical computer tomography.

This embodiment is similar to that of FIG. 17 with the exception thatthe mixed light is intermitted by a: chopper 77 and detection of thebeat component is synchronized with an intermittence cycle.

Referring to FIG. 19, there is shown an embodiment of the optical unitmade up of a bundle of a plurality of optical elements, each having alens, on the focal plane of which a Fraunhofer diffraction is formed,thereby reducing the length of the optical system. Photomixed light ofreceived light with local oscillation light is allowed to enter each ofthe optical elements. It is thus possible to detect the beat componentwith a relatively short optical system and obtain an optical tomogram ofhigh resolution.

FIG. 20 is a diagrammatic sketch illustrating an embodiment wherein animage of such an object as a living body is observed with theheterodyning and high-directionality optical system according to thisinvention.

Light from a laser light source 181 is divided by a half mirror into twolaser beams. One laser beam is directed onto an absorber 170a buriedbetween scatterers 170b and 170c, while the other laser beam isfrequency-shifted through a phase shifter for photomixing with thetransmitted beam. The resulting beam is directed through ahigh-directionality optical system 100 built of a bundle of a pluralityof receptor elements according to this invention, and the beat componentof the formed Fraunhofer diffraction image is detected by a detector280. With such an arrangement, it is possible to observe an opticaltomogram of the human body or the like with high resolution.

FIG. 21 is a diagrammatic sketch illustrating one embodiment of thearrangement for imaging optical tomograms, in which the optical systemof this invention is incorporated.

Laser light from an He-Ne laser 200 is divided by a half mirror 201 intotwo laser beams, which are then frequency-modulated by acoustoopticmodulators 203 and 204 driven by modulators 205 and 206 to make afrequency difference f between them. Then, a beam leaving the modulator204 is directed through an objective 208 onto a sample 212 driving by apulse stage 212'. The transmitted beam is photomixed with a beam passingthrough an objective 207 and a mirror 209 by a beam splitter 213,entering a high-directionality optical system 214, whence it isprocessed in a receptor 215, amplified by an amplifier 216 andspectrum-analyzed in a spectral analyzer 217 and processed through afilter 218 having a zone f to detect the beat component. The beatcomponent bears information about a transmission image buried in thescattered components. While the sample is moved by the pulse stage 212',the beat component is detected and image-processed in a computer 200 todisplay an optical tomogram on a CRT 219. If required, this image isprinted out with a printer 221.

According to this invention as mentioned above, signal light and localoscillation light are photomixed together, and the beat component of theresulting light is detected, whereby any higher order component can becut off, even when a spectrum n times as large as the 0 order spectrumis extracted out of Fraunhofer diffraction images. In other words, sinceit is difficult to extract the 0 order Fraunhofer diffraction imagealone due to its too small a size, heterodyning is used with a pinholehaving an increased diameter, thereby fixing the aperture of thereceptor system at a practical value.

INDUSTRIAL APPLICABILITY

The present invention, according to which only information light can bepicked out of scattered components as mentioned above, is applicable tooptical computer tomography, etc. If this invention is applied to thehuman body or the like, it is then possible to view only a vascularimage of the human body by using a wavelength corresponding to theabsorption region of hemoglobin, to make observation of an image of anervous system by using light at a wavelength corresponding to theabsorption wavelength of the nervous system, or to make clear images ofspecific cells having a given absorption wavelength such as cerebral orbone cells by illuminating them with light having that absorptionwavelength. Thus, this invention makes breakthroughs in medical or likeother techniques.

What is claimed is:
 1. A heterodyning receptor system, comprising:afirst laser light source supplying a first laser light transmittedthrough a sample; a second laser light source supplying a second laserlight having a frequency different from that of said laser lightproduced by said first laser light source; photomixing means forphotomixing said first laser light transmitted through the sample withsaid second laser light; a receptor element which the resulting lightenters and divides a light propagating zone into a plurality ofsub-zones; and a detector means for detecting a beat component of theresulting photomixed light out of light leaving said receptor element;said receptor element having an exit end, at which a spatial zone, whichis defined between different points and in which interference occurs, islimited within a spatially resolvable minimum unit to detect the beatcomponent of the photo mixed light.
 2. A receptor system as claimed inclaim 1, wherein said spatially resolvable minimum unit is limited bydetecting a diffraction image at most n times as large as the 0 orderspectrum of a Fraunhofer diffraction image, where n is an integergreater than 1, at the exit end of said receptor element.
 3. A receptorsystem as claimed in claim 1, wherein said receptor element detects thewhole or a part of the 0 order diffraction image of a Fraunhoferdiffraction image at the exit end of said receptor element.
 4. Areceptor system as claimed in any one of claims 1 to 3, wherein saidreceptor element is made up of an elongated pipe having pinholes at itsentrance and exit ends.
 5. A receptor system as claimed in any one ofclaims 1 to 3, wherein said receptor element is made up of a hollow,elongated pipe coated on its wall face with a light absorbing material.6. A receptor system as claimed in any one of claims 1 to 3, whereinsaid receptor element is made up of a straight optical fiber having alongitudinal axis, a core portion and a clad portion, said core portionhaving a smaller index of refraction than said clad portion, such thatlight entering said optical fiber at an angle less than a particularangle to the axis of said optical fiber is transmitted along saidoptical fiber and all other light is entering said optical fiber isscattered out of said core portion.
 7. A receptor system as claimed inany one of claims 1 to 3, wherein said receptor element includes along-focus lens whose front and back focuses are located at its entranceand exit ends, respectively.
 8. A receptor system as claimed in any oneof claims 1 to 3, wherein said receptor element includes an objectivewhose front focal position is located at said sample and an eyepiecewhose front focal position is located at the back focal position of saidobjective.
 9. An arrangement for visualizing optical transmissionimages, characterized by including a stage for moving a sample, meansfor directing one of two laser beams with a given frequency differencetherebetween onto said sample and photomixing the transmitted light withthe other laser beam, a receptor element which the resulting lightenters and divides a light propagating zone into a plurality ofsub-zones, said receptor element having an exit end, at which a spatialzone, which is defined between different points and in whichinterference occurs, is limited within a spatially resolvable minimumunit, and a detector for detecting a beat component of the mixed lightout of light leaving said receptor element, means for computing thedetected signals, and means for displaying the result of said computing,whereby a beat component is extracted from light leaving said sample tovisualize an optical transmission image.
 10. An arrangement forvisualizing optical transmission images as claimed in claim 9,characterized in that said spatially resolvable minimum unit is limitedby detecting a diffraction image at most n times as large as the 0 orderspectrum of a Fraunhofer diffraction image at the exit end of saidreceptor element.
 11. An arrangement for visualizing opticaltransmission images as claimed in claim 9, wherein said spatiallyresolvable minimum unit is limited by detecting a diffraction image atmost n times as large as the 0 order spectrum of a Fraunhoferdiffraction image, where n is an integer greater than 1, at the exit endof said receptor element.
 12. An arrangement for visualizing opticaltransmission images, comprising:a stage for moving a sample; means fordirecting one of two laser beams with a given frequency differencetherebetween onto said sample and photomixing the transmitted light withthe other laser beam; a receptor element which the resulting lightenters and divides a light propagating zone into a plurality ofsub-zones; said receptor element having an exit end, at which a spatialzone, which is defined between different points and in whichinterference occurs, is limited within a spatially resolvable minimumunit; a detector for detecting a beat component of the mixed light outof light leaving said receptor element; means for computing the detectedsignals; and means for displaying the result of said computing; wherebya beat component is extracted from light leaving said sample to producea visible optical transmission image.